The present invention relates to a method for stabilizing a tunnel junction component and a corresponding stabilized tunnel junction component, in which method a mask is formed on the surface of a substrate and conductors are built on the substrate by evaporation and at least one thin oxide layer element is oxidized on top of a selected conductor and remains partly under the next conductor, thus forming a tunnel junction element between these conductors, and which element is provided with contacts and cased.
U.S. Pat. Nos. 5,947,601 and 5,974,806 disclose certain thin film constructions exploiting tunnel junctions. It has been observed that ambient air often has a corrosive effect on nano and less-than-micron-sized thin film conductors. The long-term instability is largely due to the large surface-area/volume ratio of these small structures. This problem acquires central importance with the increase in the number of nano-electronic sensors, which should remain stable for a number of years. This instability has been quite clearly observed in the xe2x80x98Coulomb blockadexe2x80x99 nano thermometer (CBT) and micro cooler disclosed in the publications referred to above. It has become apparent that normal hermetically sealed cases for microelectronic sensors and other components are unsuitable and insufficient in these cases, so that the aging problem requires a more developed solution.
The variation in the resistance of a CBT sensor signifies an increase in dispersion in the junction parameters, thus reducing the absolute accuracy. A relative change of 10% in the resistance of the sensor has been observed to cause a maximum change of 0.5% in absolute accuracy.
Sensors and other components of this kind cannot generally be installed in metal capsules, as this increases the size and mass, as well as demanding non-magnetic materials and involving a risk of overheating when closing the capsule. It is also difficult to use protective gases, because an extremely high degree of purity would be required, which is difficult to maintain in a small volume. It is also difficult to create and maintain a sufficient vacuum in a small volume.
It is difficult to use a coating substance to protect a component, as the protection cannot be carried out before removing the evaporation mask, which requires the component to be taken from the evaporation chamber to an air space, which would mean the continuation of aging after manufacture, despite the coating protection.
In the above publications, a line width of typically 200 nm, generally 0.01-10 micrometers (10-10 000 nm), is used in the tunnel junction. In the thermometer, the length of the tunnel junction is typically 300 nm. The thickness of the oxide forming a tunnel junction is extremely small, typically only about 1 nm. This is considerably less than the sizes, for example, in the P and N elements doped in a silicon chip, which are used in conventional microcircuits (memories, processors, and digital and analog circuits in general).
The present invention provides a manufacturing method for stabilizing tunnel junction components and a stabilized tunnel junction component, which will solve the aging problem. The characteristic features of a manufacturing method according to the invention in which a mask is formed on the surface of a substrate, and conductors are constructed by evaporation onto the substrate in an evaporation chamber, and at least one thin oxide layer element is oxidized on top of a selected conductor, which remains partly under the following conductor, thus forming a tunnel junction element with those conductors, and which tunnel junction component is provided with contacts and cased, is characterized in that titanium (Ti) or another gettering substance is vaporized on top of the said following conductor, before the tunnel junction component is removed from the evaporation chamber, when the titanium layer thus created protects the tunnel junction element from the detrimental effects of air molecules, both in the later stages of the manufacture of the tunnel junction component and as a finished component.
Correspondingly, the characteristic features of a tunnel junction according to the invention in which there are conductors on the surface of the substrate and at least one oxide layer between two overlapping conductors, thus forming a tunnel junction element with them, and in which the tunnel junction component includes contacts to connect it to external circuits, is characterized in that there is a titanium layer (Ti) on top of the uppermost conductor.
According to the invention, the tunnel junction can be protected immediately after manufacture, before the tunnel junction component is removed from the evaporation chamber. The titanium layer appears to have the effect that the thin oxide layer is no longer able to grow, on account of the air molecules, especially the oxygen and water molecules, because the titanium layer gathers the molecules into itself. Some other substance, particularly a metal, with similar gettering properties can be used. The reactivity of the getter must be essentially greater than that of the substance used in the conductor layer. In the same way as titanium (Ti), the other substance absorbs the oxygen and water molecules in the air into itself. However, it is important that the materials used in manufacture and the uncased component are not exposed to moisture.
According to one preferred application, the uppermost conductor is oxidized before the titanium layer is evaporated, so that the titanium layer cannot substantially affect the electrical values of the circuit. According to a second preferred application, the titanium layer is coated already in the same stage with a layer of copper or other metal, so that the titanium layer does not become saturated when the tunnel junction component must be removed from the evaporation chamber to remove the mask.